The present invention relates to the art of power driven threading machines and, more particularly, to an improved thread cutting die head construction, and improved control of receding thread cutting dies in connection with the cutting of a tapered thread on a workpiece.
Power driven thread cutting machines are of course well known and basically comprise a rotatable spindle and chuck assembly for rotating a workpiece to be threaded, and a thread cutting die head supported on a tool carriage for axial displacement relative to the workpiece to advance thread cutting dies supported by the die head into thread cutting engagement with the end of the workpiece. Generally, the die head assembly is pivotally supported on one side of the tool carriage for displacement between stored and use positions. In the use position the axis of the die head is aligned with the workpiece axis, and the side of the die head opposite the pivotal mounting engages the corresponding side of the tooling carriage to axially and vertically support the die head in the use position.
Receding die heads are likewise known and operate to cut a tapered thread on a workpiece by providing for the thread cutting dies to progressively recede radially relative to the workpiece during axial advancement of the cutting dies in thread cutting engagement with the workpiece. In such a receding die head, radial receding movement of the cutting dies during the thread cutting operation has been achieved heretofore in a manner shown for example in U.S. Pat. No. 4,023,211 to Miyagawa. More particularly in this respect, the thread cutting dies are supported on a plate member of the die head assembly which is pivotal about the die head axis. The pivotal plate member has a radial arm which engages a ramp or cam component supported on the carriage assembly. The torque developed by engagement of the cutting dies with a workpiece pivotally displaces the plate member for the arm to force the ramp component downwardly against an underlying carriage support rail. Thereafter, the carriage and die head move relative to the ramp with the cutting dies in cutting engagement with the workpiece, and the arm of the pivotal plate member follows the contour of the ramp as a result of the torque force. Accordingly, the pivotal plate pivots relative to the die head during axial advancement thereof, and such relative pivotal displacement causes the cutting dies to progressively recede radially during the thread cutting operation. When the arm reaches the end of the ramp and drops therefrom, the cutting tools are rapidly displaced radially outwardly to clear the workpiece and terminate the threading cutting operation.
Die head structures of the foregoing character and control of the receding displacement of thread cutting dies in the manner disclosed in the Miyagawa patent are disadvantageous for a number of reasons. For example in this respect, the torque developed during engagement of the thread cutting dies and workpiece is erratic, whereby positive displacement of the ramp component into stable engagement with the underlying carriage support rail at the beginning of the operation is not assured. Further, fixed axial positioning of the ramp component relative to the moving carriage is necessary, and such positioning is likewise not assurable because of the erratic force application on the arm of the pivotal plate member. Still further, the torque generated during the cutting operation is not uniform, and this can lead to non-uniform force engagement between the pivotal plate arm and ramp and thus non-uniform wear of the engaging surfaces. Moreover, the area between the underside of the ramp and the carriage support rail is susceptible to the reception of dirt, oil and metal chips, and such foreign matter interposed between the ramp and support rail can cause wear and/or damage thereto and, more importantly, can distort the intended profile of the ramp relative to horizontal and thus the desired taper of the thread being cut. All of these disadvantages, individually or collectively, affect the ability to achieve accurate thread cutting and/or consistently uniform thread cutting. A further disadvantage with the arrangement as disclosed in the Miyagawa patent resides in the fact that a tapered thread can be cut in only one direction on a workpiece, namely that direction in which the torque generated will result in displacement of the ramp downwardly against the carriage support rail.